U.S. patent application number 12/358344 was filed with the patent office on 2009-07-30 for piezoelectric thin film device.
This patent application is currently assigned to Hitachi Cable, Ltd.. Invention is credited to Fumihito Oka, Kenji SHIBATA.
Application Number | 20090189482 12/358344 |
Document ID | / |
Family ID | 40898493 |
Filed Date | 2009-07-30 |
United States Patent
Application |
20090189482 |
Kind Code |
A1 |
SHIBATA; Kenji ; et
al. |
July 30, 2009 |
Piezoelectric Thin Film Device
Abstract
A piezoelectric thin film device according to the present
invention comprises a lower electrode, a piezoelectric thin film
and an upper electrode, in which the piezoelectric thin film is
formed of an alkali niobium oxide-based perovskite material
expressed by (K.sub.1-xNa.sub.x)NbO.sub.3 (0<x<1), and in
which dependency of the piezoelectric constant d.sub.31 of the
piezoelectric thin film on applied electric field [=|(d.sub.31
under 70 kV/cm)-(d.sub.31 under 7 kV/cm)|/|d.sub.31 under 70
kV/cm|] is 0. 20 or less.
Inventors: |
SHIBATA; Kenji; (Tsukuba,
JP) ; Oka; Fumihito; (Tsuchiura, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Assignee: |
Hitachi Cable, Ltd.
|
Family ID: |
40898493 |
Appl. No.: |
12/358344 |
Filed: |
January 23, 2009 |
Current U.S.
Class: |
310/311 |
Current CPC
Class: |
H01L 41/319 20130101;
G01C 19/5663 20130101; H01L 41/094 20130101; H01L 41/1136 20130101;
H01L 41/1873 20130101; H01L 41/0815 20130101; H01L 41/316
20130101 |
Class at
Publication: |
310/311 |
International
Class: |
H01L 41/187 20060101
H01L041/187 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2008 |
JP |
2008-013974 |
Claims
1. A piezoelectric thin film device, comprising: a lower electrode,
a piezoelectric thin film and an upper electrode, wherein: the
piezoelectric thin film is formed of an alkali niobium oxide-based
perovskite material expressed by (K.sub.1-xNa.sub.x)NbO.sub.3
(0<x<1), and wherein: dependency of the piezoelectric
constant d.sub.31 of the piezoelectric thin film on applied
electric field[=|(d.sub.31 under 70 kV/cm)-(d.sub.31 under 7
kV/cm)|/|d.sub.31 under 70 kV/cm|] is 0.20 or less.
2. The piezoelectric thin film device according to claim 1,
wherein: the piezoelectric thin film has a strong (001).sub.KNN
plane diffraction peak which occupies 80% or more of diffraction
peaks of the piezoelectric thin film in an X-ray diffraction
2.theta./.theta. measurement to a surface of the piezoelectric thin
film.
3. The piezoelectric thin film device according to claim 1,
wherein: the lower electrode is formed of a platinum thin film.
4. The piezoelectric thin film device according to claim 1,
wherein: between the lower electrode and the piezoelectric thin
film is interposed a thin film of a material selected from a group
consisting of LaNiO.sub.3, NaNbO.sub.3 and
(K.sub.1-xNa.sub.x)NbO.sub.3 (0<x<1) having a composition
ratio x greater than that of the piezoelectric thin film.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority from Japanese patent
application serial no. 2008-013974 filed on Jan. 24, 2008, the
content of which is hereby incorporated by reference into this
application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to piezoelectric thin film
devices using a piezoelectric thin film, more specifically to
piezoelectric thin film devices including, on an Si (silicon)
substrate, a piezoelectric thin film of an alkali niobium
oxide-based perovskite material.
[0004] 2. Description of Related Art
[0005] Piezoelectric materials are used for piezoelectric devices
of various applications. For example, they are widely used for
functional electronic components such as actuators in which an
applied voltage deforms a piezoelectric element thereby providing
an actuation function, and sensors for detecting a physical
quantity by utilizing, conversely to actuators, a voltage generated
by a deformation of a piezoelectric element. As piezoelectric
materials for use in such actuators and sensors, there have been
widely used lead-based dielectric materials with excellent
piezoelectric properties, in particular perovskite structure
ferroelectric materials expressed by the general chemical formula:
Pb(Zr.sub.1-xTi.sub.x)O.sub.3 (often called PZTs). A PZT is
typically made by sintering an oxide of its constituent metals.
[0006] In the trend toward downsizing and increasing performance of
electronic components, there is also a strong demand for
piezoelectric devices with smaller size and higher performance.
However, as a piezoelectric material made by widely used
conventional sintering methods becomes thinner, the following
problem comes to the fore. Specifically, as the thickness of a
piezoelectric material approaches the order of 10 .mu.m, it becomes
comparable to the grain size of the piezoelectric material;
therefore, the influence of the grain boundaries can no longer be
ignored. This produces problems such as fluctuation in
piezoelectric properties and accelerated device degradation. In
order to solve such problems by replacing conventional sintering
methods, fabrication methods of piezoelectric materials such as
those utilizing thin film formation techniques have been researched
in recent years. Therefrom, there have been reported PZT films
sputtered on an Si substrate for use in high-sensitivity gyro
sensors (angular velocity sensors) (e.g., see
JP-A-2005-203725).
[0007] On the other hand, PZT piezoelectric sintered bulks and PZT
piezoelectric thin films contain approximately 60 to 70 mass % of
lead; so, it is desired to promote research and development of
lead-free piezoelectric materials from an environmental
consideration. Various lead-free piezoelectric materials are
currently being studied, among which is potassium sodium niobate
expressed by the general chemical formula:
(K.sub.1-xNa.sub.x)NbO.sub.3 (0<x<1) (hereinafter also
referred to as KNN). A KNN has a perovskite structure and exhibits
relatively excellent piezoelectric properties among lead-free
piezoelectric materials, and is therefore expected to be a
promising lead-free piezoelectric material candidate. A KNN
piezoelectric material has excellent piezoelectric properties near
x=0.5. And, there is a report that a KNN thin film epitaxially
formed on an MgO single crystalline substrate (instead of an Si
substrate) exhibits good piezoelectric properties (see Nonpatent
Document 1).
[0008] Nonpatent Document 1: T. Mino, S. Kuwajima, T. Suzuki, I.
Kanno, H. Kotera, and K. Wasa: Jpn. J. Appl. Phys., 46 (2007)
6960.
[0009] Such KNN thin films have been attempted to be formed on an
Si substrate by other film formation methods such as sputtering and
PLD (pulsed laser deposition). However, up to now, KNN thin films
on an Si substrate exhibit a relatively low piezoelectric constant
d.sub.31 compared to PZT thin films, and therefore have yet to be
applied to high-sensitivity sensors such as gyro sensors. Moreover,
in the above Nonpatent Document 1, the KNN piezoelectric thin film
is formed on an MgO substrate; however, use of MgO substrates
presents a cost disadvantage since they are expensive compared to
silicon substrates.
SUMMARY OF THE INVENTION
[0010] Under these circumstances, the present invention addresses
the above problems. It is an objective of the present invention to
provide a piezoelectric thin film device using a KNN thin film
formed on an Si substrate which has sufficiently high performance
to be applied to gyro sensors and the like.
[0011] In order to achieve the objective described above, the
present invention is configured as follows.
[0012] According to one aspect of the present invention, a
piezoelectric thin film device comprises a lower electrode, a
piezoelectric thin film and an upper electrode, in which the
piezoelectric thin film is formed of an alkali niobium oxide-based
perovskite material expressed by (K.sub.1-xNa.sub.x)NbO.sub.3
(0<x<1), and in which dependency of the piezoelectric
constant d.sub.31 of the piezoelectric thin film on applied
electric field [=|(d.sub.31 under 70 kV/cm)-(d.sub.31 under 7
kV/cm)|/|d.sub.31 under 70 kV/cm|] is 0.20 or less.
[0013] In the above aspect of the present invention, the following
improvements and modifications can be made.
[0014] (i) The piezoelectric thin film has a strong (001).sub.KNN
plane diffraction peak which occupies 80% or more of diffraction
peaks of the piezoelectric thin film in an X-ray diffraction
2.theta./.theta. measurement to a surface of the piezoelectric thin
film.
[0015] (ii) The lower electrode is formed of a platinum (Pt) thin
film.
[0016] (iii) Between the lower electrode and the piezoelectric thin
film is interposed a thin film of a material selected from a group
consisting of LaNiO.sub.3, NaNbO.sub.3 and
(K.sub.1-xNa.sub.x)NbO.sub.3 (0<x<1) having a composition
ratio x greater than that of the piezoelectric thin film.
Advantages of the Invention
[0017] According to the present invention, it is possible to
provide a piezoelectric thin film device using a KNN thin film
formed on an Si substrate which can greatly increase the
piezoelectric constant d.sub.31 under lower applied electric fields
by suppressing the dependency of the piezoelectric constant
d.sub.31 on applied electric field to low levels, thus providing
sufficiently high performance to be applied to gyro sensors and the
like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic illustration showing a cross-sectional
view of a piezoelectric thin film device according to an embodiment
of the present invention.
[0019] FIGS. 2(a) and 2(b) are schematic illustrations for
explaining a measurement method of the piezoelectric constant
d.sub.31 of a piezoelectric thin film device.
[0020] FIG. 3 is a schematic illustration showing a cross-sectional
view of the piezoelectric thin film device of Examples 1 to 4 and
Comparative examples 1 to 6.
[0021] FIG. 4 is a table showing, for Examples 1 to 4 and
Comparative examples 1 to 6, forming condition of Pt/Ti film,
occupation ratio of (001).sub.KNN plane, and piezoelectric
properties of the KNN thin film.
[0022] FIG. 5 is an example of a diffraction pattern by an X-ray
diffraction 2.theta./.theta. measurement to a surface of the
piezoelectric thin film device of Comparative example 1.
[0023] FIG. 6 is an example of a diffraction pattern by an X-ray
diffraction 2.theta./.theta. measurement to a surface of the
piezoelectric thin film device of Example 1.
[0024] FIG. 7 shows a relationship between piezoelectric constant
d.sub.31 and applied electric field for piezoelectric thin film
devices of Example 1 to 4 and Comparative example 1 to 6.
[0025] FIG. 8 is a schematic illustration showing a cross-sectional
view of the piezoelectric thin film device of Examples 5 to 8.
[0026] FIG. 9 is a table showing, for Examples 5 to 8, type of an
orientation control layer, and occupation ratio of (001).sub.KNN
plane and piezoelectric properties of the KNN thin film.
[0027] FIG. 10 shows a relationship between piezoelectric constant
d.sub.31 and applied electric field for piezoelectric thin film
devices of Examples 5 to 8.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] As described above, conventional KNN piezoelectric thin
films on an Si substrate did not have sufficient piezoelectric
constant d.sub.31 to be used for gyro sensors. Typically, the
piezoelectric constant d.sub.31 of a piezoelectric thin film is
relatively low under lower applied electric fields, while it is
considerably higher under higher applied electric fields than under
lower applied electric fields. That is, the piezoelectric constant
d.sub.31 of a piezoelectric thin film generally has a strong
dependency on applied electric field and tends to increase with
increasing applied electric field (i.e., the gradient of the
piezoelectric constant d.sub.31 with respect to applied electric
field is positive and large).
[0029] Actually, the fact that piezoelectric thin films have low
piezoelectric constants d.sub.31 under lower applied electric
fields has been a limiting factor in improving the sensitivity of
gyro sensors. Moreover, it is conventionally regarded that such an
above-described dependency of the piezoelectric constant d.sub.31
of a piezoelectric thin film on applied electric field is an
essential phenomenon and therefore there is no solution.
[0030] Through research and development of strongly preferentially
(001).sub.KNN oriented KNN films on an Si substrate by the
inventors, it has been achieved a KNN piezoelectric thin film
having a piezoelectric constant d.sub.31 with a weak dependency on
applied electric field. In further study of thus obtained
piezoelectric thin films, it has been revealed that, by suppressing
the dependency of the piezoelectric constant d.sub.31 on applied
electric field [=|(d.sub.31 under 70 kV/cm)-(d.sub.31 under 7
kV/cm)|/|d.sub.31 under 70 kV/cm|] to 0.20 or less, the
piezoelectric constant d.sub.31 (particularly that under relatively
low applied electric fields) can be greatly increased compared to
those of conventional piezoelectric thin films (which will be
detailed later with reference to e.g. FIG. 7).
[0031] By using such a lead-free KNN thin film on an Si substrate
having such excellent characteristics, a piezoelectric thin film
device can be provided with sufficient properties to be applicable
to gyro sensors or the like, which conventional arts have had
difficulty in providing. In addition, the use of an Si substrate
enables the piezoelectric thin film device of the present invention
to be readily integrated with semiconductor control circuits
therefor or other semiconductor circuits and devices on the same
substrate.
[0032] A piezoelectric thin film device according to a preferred
embodiment of the present invention will be described below with
reference to the accompanying drawings. However, the present
invention is not limited to the embodiments described herein.
[0033] FIG. 1 is a schematic illustration showing a cross-sectional
view of a piezoelectric thin film device according to an embodiment
of the present invention. As shown in FIG. 1, the piezoelectric
thin film device 10 of this embodiment is fabricated by
sequentially forming, on an Si substrate 1, a lower electrode 2, a
KNN piezoelectric thin film 3 and an upper electrode 4.
[0034] The Si substrate 1 is an Si single crystalline substrate
having a (100).sub.Si oriented surface (hereinafter "(100) Si
substrate"). The Si substrate 1 may have an oxide film (SiO.sub.2)
formed on its surface in order to electrically insulate the lower
electrode 2 and Si substrate 1.
[0035] The lower electrode 2 serves as an important underlayer for
forming the KNN piezoelectric thin film 3 thereon, and therefore it
is preferable to employ Pt (platinum) as the electrode material.
This is because Pt films formed on the Si substrate 1 are
self-oriented preferentially to a (111).sub.Pt plane. In this
embodiment, the lower electrode 2 was formed of a Pt thin film
grown by RF (radio frequency) magnetron sputtering. In addition, it
is more preferable to provide a Ti (titanium) adhesive layer
between the Si substrate 1 and lower electrode 2 in order to
enhance the adhesiveness of the lower electrode 2 (see FIG. 3,
details are described later).
[0036] Unlike the lower electrode 2, the upper electrode 4, which
is formed on the KNN piezoelectric thin film 3, does not affect
qualities of the piezoelectric film 3. Therefore, there is no
particular limitation on the electrode material used. In this
embodiment, similarly to the lower electrode 2, the upper electrode
4 was formed of a Pt thin film grown by RF magnetron
sputtering.
[0037] The KNN piezoelectric thin film 3 is made of an alkali
niobium oxide-based perovskite material expressed by the general
chemical formula (K.sub.1-xNa.sub.x)NbO.sub.3 (0<x<1).
Preferably, the composition x(=Na/[K+Na]) is approximately 0.5. The
KNN piezoelectric thin film 3 can be formed by sputtering, CVD,
PLD, sol-gel process, etc. In this embodiment, the KNN
piezoelectric thin film 3 was formed by RF magnetron sputtering.
And, occupation ratio of the (001).sub.KNN plane diffraction of the
KNN piezoelectric thin film 3 is preferably 80% or more in an X-ray
diffraction 2.theta./.theta. measurement to a surface of the
piezoelectric thin film 3. In addition, to the KNN piezoelectric
thin film 3 of this embodiment may be added any one of Ta, Li and
Sb, or any combination thereof.
[0038] Herein, an evaluation (measurement) method for a state of a
crystal grain alignment of the piezoelectric thin film by X-ray
diffraction (XRD) is to be described. In an XRD 2.theta./.theta.
measurement, a specimen and a detector are scanned by the .theta.
axis, wherein a scanning angle of the specimen is .theta. and that
of the detector is 2.theta.. According to the 2.theta./.theta.
measurement, it can be estimated which crystal plane is a
predominant plane at a surface of the piezoelectric thin film.
Occupation ratio of the (001).sub.KNN plane of a KNN piezoelectric
thin film is determined using diffraction peaks of KNN positioned
at an angle 2.theta. between 20.degree. and 38.degree. in the
2.theta./.theta. measurement. Specifically, the occupation ratio of
the (001).sub.KNN plane is defined as below:
Occupation ratio of (001).sub.KNN plane
(%)=[I.sub.(001)KNN/{I.sub.(001)KNN+I.sub.(110)KNN}].times.100
[0039] in which
[0040] I.sub.(001)KNN: diffraction peak intensity of (001).sub.KNN
plane;
[0041] I.sub.(110)KNN: diffraction peak intensity of (110).sub.KNN
plane.
[0042] Here, the inventors consider that the X-ray diffraction peak
positioned at an angle 2.theta. between 22.011.degree. and
22.890.degree. can be attributed to the (001).sub.KNN plane.
Diffraction peaks due to the Si substrate and lower electrode are
excluded from calculation of the occupation ratio of the KNN thin
film. Also, it is in order to ensure the exclusion of diffraction
peaks such as (002).sub.KNN plane and (111).sub.Pt plane from the
calculation that the diffraction angle 2.theta. is limited to a
range between 20.degree. and 38.degree.. Furthermore, the X-ray
diffraction in the present invention is always conducted by using
the Cu--K.alpha. ray.
[0043] On the other hand, the dependency of the piezoelectric
constant d.sub.31 of the KNN piezoelectric thin film 3 on applied
electric field (kV/cm) is defined by the expression: |(d.sub.31
under 70 kV/cm)-(d.sub.31 under 7 kV/cm)|/|d.sub.31 under 70
kV/cm|, i.e., absolute value of difference between "d.sub.31 under
70 kV/cm" and "d.sub.31 under 7 kV/cm" is divided by absolute value
of "d.sub.31 under 70 kV/cm". The piezoelectric thin film 3 is
formed so that this dependency value is 0.20 or less.
[0044] The piezoelectric constant d.sub.31 of the piezoelectric
thin film 3 will be now described with reference to FIGS. 2(a) and
2(b). FIGS. 2(a) and 2(b) are schematic illustrations for
explaining a measurement method of the piezoelectric constant
d.sub.31 of a piezoelectric thin film device.
[0045] Firstly, a rectangular strip is cut from the Si substrate 1
in FIG. 1 to fabricate an elongated piezoelectric thin film device
10. Next, one end of the piezoelectric thin film device 10 is
clamped with a clamp 20 and the other end is open to configure a
simplified unimorph cantilever (FIG. 2(a)). Then, the KNN
piezoelectric thin film 3 is stretched or compressed by applying a
voltage between the upper electrode 4 and lower electrode 2,
thereby causing the entire cantilever (piezoelectric thin film
device 10) to bend. And, the displacement .DELTA. in the vertical
direction (the thickness direction of the piezoelectric film 3) at
the other end (open end) of the cantilever is measured using a
laser Doppler displacement meter 21 (FIG. 2(b)).
[0046] The piezoelectric constant d.sub.31 is calculated from the
displacement .DELTA., the cantilever length, the thicknesses and
Young's moduli of the substrate 1 and piezoelectric thin film 3 and
the applied electric field(=[applied voltage]/[film thickness]).
For details on the d.sub.31 calculation formula, see reference: I.
Kanno, H. Kotera, and K. Wasa: Measurement of transverse
piezoelectric properties of PZT thin films, Sens. Actuators A 107
(2003) 68.
[0047] The dependency of the piezoelectric constant d.sub.31 on
applied electric field is determined by varying the electric field
applied to the piezoelectric thin film 3 of the cantilever. That
is, the dependency of the piezoelectric constant d.sub.31 on
applied electric field[=|(d.sub.31 under 70 kV/cm)-(d.sub.31 under
7 kV/cm)|/|d.sub.31 under 70 kV/cm|] can be calculated using the
d.sub.31 values under 70 kV/cm and 7 kV/cm.
[0048] By suppressing the dependency of the piezoelectric constant
d.sub.31 of a KNN thin film on applied electric field to 0.20 or
less, the piezoelectric constant d.sub.31, particularly that under
relatively low applied electric fields (e.g., 7 kV/cm), can be
greatly increased (see FIGS. 4 and 5, details are described later)
and, as a result, there can be realized a gyro sensor sensitivity
comparable to that of gyro sensors using a conventional PZT thin
film.
[0049] Further, a piezoelectric KNN thin film having a smaller
dependency of the piezoelectric constant d.sub.31 on applied
electric field offers an advantage that, when used as an actuator,
the input voltage (or the input electric field) is nearly
proportional to the displacement, and therefore no additional
control circuits are required. Such a piezoelectric KNN thin film
has another advantage of having piezoelectric properties resistance
to deterioration with age and therefore a longer service life.
[0050] A KNN thin film having a smaller dependency of the
piezoelectric constant d.sub.31 on applied electric field can be
achieved by increasing a (001).sub.KNN plane orientation preference
thereof. In order to suppress the dependency of the piezoelectric
constant d.sub.31 on applied electric field to 0.20 or less, the
occupation ratio of (001).sub.KNN plane diffraction of the KNN thin
film in the XRD 2.theta./.theta. measurement is preferably 80% or
more. A KNN piezoelectric thin film 3 with a stronger (001)
orientation preference can be obtained by, for example, using a
highly preferentially (111).sub.Pt plane oriented Pt thin film as
the lower electrode 2 underlying the KNN film 3. The highly
preferentially (111).sub.Pt plane oriented Pt thin film can be
achieved by, for example, making thinner a Ti adhesive layer formed
between the Pt thin film and Si substrate, forming the Pt thin film
at higher temperatures, or sputtering the Pt thin film in an
ambient with lower O.sub.2 partial pressure.
[0051] A KNN piezoelectric thin film 3 having a smaller dependency
of the piezoelectric constant d.sub.31 on applied electric field
can also be achieved by interposing, between the Pt lower electrode
2 and KNN piezoelectric film 3, an orientation control layer such
as a LaNiO.sub.3 thin film, a NaNbO.sub.3 thin film and an Na rich
(K.sub.1-xNa.sub.x)NbO.sub.3 thin film having a composition ratio x
greater than that of the KNN piezoelectric thin film 3. The
orientation control layer is for enhancing the (001).sub.KNN
orientation preference of the KNN piezoelectric thin film 3 formed
on the Pt lower electrode 2. By forming such a film (e.g., a
LaNiO.sub.3 thin film) on the lower electrode 2, a KNN film formed
thereon can be made to exhibit a stronger (001).sub.KNN orientation
preference than one formed directly on the Pt lower electrode
2.
[0052] Referring to FIG. 1 again, a sensor for detecting a physical
quantity can be obtained by at least connecting a voltage detecting
means between the lower electrode 2 and upper electrode 4.
Deformation of the piezoelectric thin film device of this sensor
due to a change in some physical quantity will generate a
corresponding voltage; thus, various physical quantities can be
detected by sensing such voltage. On the other hand, an actuator
can be obtained by at least connecting a voltage applying means
between the lower electrode 2 and upper electrode 4 in FIG. 1. A
voltage application to this sensor will deform the piezoelectric
thin film device, thereby enabling actuation of various members.
Such sensors include gyro sensors, supersonic sensors, pressure
sensors, and velocity/acceleration sensors. And, such an actuator
can be used, e.g., in inkjet printers, scanners and supersonic
generators.
EXAMPLES
[0053] Examples of the invention will be described below, however
the present invention is not limited by these examples.
Examples and Comparative Examples of Piezoelectric Thin Film Device
Having Structure in FIG. 3
[0054] FIG. 3 is a schematic illustration showing a cross-sectional
view of the piezoelectric thin film device of Examples 1 to 4 and
Comparative examples 1 to 6. The piezoelectric thin film devices 30
of Examples 1 to 4 and Comparative examples 1 to 6 were fabricated
by sequentially forming, on an Si substrate 11 (having an SiO.sub.2
film 15 on its surface), a Ti adhesive layer 16, a Pt lower
electrode 12, a (K.sub.0.5Na.sub.0.5)NbO.sub.3 piezoelectric thin
film 13, and a Pt upper electrode 14.
[0055] Next, the fabrication method of the piezoelectric thin film
device of Examples 1 to 4 will be detailed.
[0056] As the Si substrate 11, there was used an Si substrate with
a thermal oxide layer (an SiO.sub.2 film 15) on the substrate
surface ((100).sub.Si single crystalline substrate of 4-inch round
wafer, substrate thickness of 0.5 mm, SiO.sub.2 layer thickness of
0.5 .mu.m). Firstly, on the Si substrate 11 was sequentially formed
the Ti adhesive layer 16 (thickness of 1 to 3 nm) and the Pt lower
electrode 12 (exclusively (111).sub.Pt oriented, thickness of 0.2
.mu.m) by RF magnetron sputtering. The condition of formation of
the Ti adhesive layer 16 and Pt lower electrode 12 was as follows:
substrate temperature of 350 to 400.degree. C.; discharge power of
200 W; introduced gas of Ar/O.sub.2 (Ar/O.sub.2=99/1 to 100/0);
pressure of 2.5 Pa; and formation time of 1 to 3 min (for the Ti
layer 16) and of 10 min (for the Pt electrode 12).
[0057] Subsequently, on the Pt lower electrode 12 was formed a
3-.mu.m-thick (K.sub.0.5Na.sub.0.5)NbO.sub.3 piezoelectric thin
film 13 by RF magnetron sputtering. The condition of formation of
the (K.sub.0.5Na.sub.0.5)NbO.sub.3 piezoelectric thin film 13 was
as follows: sputtering target of sintered (K, Na)NbO.sub.3
[composition ratio: (K+Na)/Nb=1, Na/(K+Na)=0.5]; substrate
temperature of 600.degree. C.; discharge power of 100 W; introduced
gas of Ar; pressure of 0.4 Pa; and film formation time of 4 h.
[0058] On the (K.sub.0.5Na.sub.0.5)NbO.sub.3 piezoelectric thin
film 13 was further formed a 0.02-.mu.m-thick Pt upper electrode 14
by RF magnetron sputtering. The condition of formation of the Pt
upper electrode 14 was as follows: without substrate heating;
discharge power of 200 W; introduced gas of Ar; pressure of 2.5 Pa;
and film formation time of 1 min.
[0059] Next, the fabrication method of the piezoelectric thin film
device of Comparative examples 1 to 6 will be detailed.
[0060] As the Si substrate 11, there was used the same conditions'
Si substrate with a thermal oxide layer 15 as was employed in
Examples 1 to 4. First, on the Si substrate 11 was sequentially
formed the Ti adhesive layer 16 (thickness of 5 to 10 nm) and the
Pt lower electrode 12 (exclusively (111).sub.Pt oriented, thickness
of 0.2 .mu.m) by RF magnetron sputtering. The formation conditions
of the Ti adhesive layer 16 and Pt lower electrode 12 were as
follows: substrate temperature of 250 to 350.degree. C.; discharge
power of 200 W; introduced gas of Ar/O.sub.2 (Ar/O.sub.2=90/10 to
98/2); pressure of 2.5 Pa; and formation time of 5 to 10 min (for
the Ti layer 16) and of 10 min (for the Pt electrode 12).
[0061] Subsequently, on the Pt lower electrode 12 was sequentially
formed a 3-.mu.m-thick (K.sub.0.5Na.sub.0.5)NbO.sub.3 piezoelectric
thin film 13 and a 0.02-.mu.m-thick Pt upper electrode 14 by RF
magnetron sputtering. The conditions of formation of the
(K.sub.0.5Na.sub.0.5)NbO.sub.3 piezoelectric thin film 13 and Pt
upper electrode 14 were the same as those used in Examples 1 to
4.
[0062] FIG. 4 is a table showing, for Examples 1 to 4 and
Comparative examples 1 to 6, forming condition of Pt/Ti film [(Pt
lower electrode 12)/(Ti adhesive layer 16)], occupation ratio of
(001).sub.KNN plane, and piezoelectric properties of the KNN thin
film. In Examples 1 to 4, in order to cause the Pt thin film to be
highly preferentially (111).sub.Pt oriented, the Ti adhesive layer
was formed thinner (e.g., 1 to 3 nm) and the Pt film was formed at
a higher temperature (e.g., 350 to 400.degree. C.) and a sputtering
ambient having a lower O.sub.2 concentration was employed (e.g., 0
to 1%) than in Comparative examples 1 to 6.
[0063] Further, in order to examine a state of a crystal grain
alignment of the (K.sub.0.5Na.sub.0.5)NbO.sub.3 (KNN) piezoelectric
thin films 13, an X-ray diffraction 2.theta./.theta. measurement
was performed for the piezoelectric thin film devices (of the above
Examples and Comparative examples) whose Pt upper electrode 14 had
not been formed and whose KNN thin film 13 was therefore exposed.
FIG. 5 is an example of a diffraction pattern by an X-ray
diffraction 2.theta./.theta. measurement to a surface of the
piezoelectric thin film device of Comparative example 1; and FIG. 6
is an example of a diffraction pattern by the X-ray diffraction
2.theta./.theta. measurement to a surface of the piezoelectric thin
film device of Example 1. Comparing FIG. 5 with FIG. 6, it can be
seen that the KNN thin film 13 of Example 1 exhibits a considerably
stronger (001).sub.KNN orientation preference (a higher occupation
ratio of (001).sub.KNN plane) than that of Comparative example
1.
[0064] In FIG. 4 is shown the occupation ratio of (001).sub.KNN
plane of the KNN thin films of Examples 1 to 4 and Comparative
examples 1 to 6. The KNN thin films of Examples 1 to 4 exhibit the
occupation ratio of (001).sub.KNN plane of as high as 80% or
more.
[0065] The piezoelectric thin film devices of the above Examples 1
to 4 and Comparative examples 1 to 6 were measured for the
piezoelectric constant d.sub.31. The measurement was done according
to the method described above in FIGS. 2(a) and 2(b). For use in
the cantilever, a 20-mm-long and 2.5-mm-wide rectangular strip of
the piezoelectric thin film device was fabricated. The
piezoelectric constant d.sub.31 was calculated using a Young's
modulus of 104 GPa for the KNN piezoelectric thin film 13.
[0066] FIG. 7 shows a relationship between piezoelectric constant
d.sub.31 and applied electric field for piezoelectric thin film
devices of Examples 1 to 4 and Comparative examples 1 to 6. In FIG.
4 are also shown gradient of the piezoelectric constant
d.sub.31[=|(d.sub.31 under 70 kV/cm)-(d.sub.31 under 7
kV/cm)|/|d.sub.31 under 70 kV/cm|]; the d.sub.31 value under 7
kV/cm; and the d.sub.31 value under 70 kV/cm as piezoelectric
properties of the KNN thin film.
[0067] For example, the gradient of the piezoelectric constant
d.sub.31 of Example 1 is calculated from: |(d.sub.31 under 70
kV/cm)-(d.sub.31 under 7 kV/cm)|/|d.sub.31 under 70 kV/cm|=|(-79
pm/V)-(-63 pm/V)|/|-79 pm/V|]=0.2.
[0068] FIGS. 4 and 7 also show the piezoelectric constant d.sub.31
of a prior art obtained for a KNN film formed on an Si substrate
(reported in: Y. Nakashima, W. Sakamoto, H. Maiwa, T. Shimura, and
T. Yogo: Jpn. J. Appl. Phys., 46 (2007) L311). The above prior art
forms the KNN film by CSD (chemical solution deposition) and
probably provides the best data reported in papers for KNN films on
Si substrates. In addition, the KNN film of the above prior art has
about 50% (001).sub.KNN oriented grains and about 50% (110).sub.KNN
oriented grains. The above prior art reports that the d.sub.33
value is 46 pm/V. In FIG. 4, the prior art d.sub.31 value is
determined to be -23 pm/V using the assumption that
"d.sub.31=-d.sub.33/2". As is apparent from Examples 1 to 4 in
FIGS. 4 and 7, the piezoelectric constant d.sub.31, particularly
that under relatively low applied electric fields (e.g., 7 kV/cm),
can be greatly increased either by suppressing the dependency of
the piezoelectric constant d.sub.31 on applied electric
field[=|(d.sub.31 under 70 kV/cm)-(d.sub.31 under 7
kV/cm)|/|d.sub.31 under 70 kV/cm|] to 0.20 or less and/or by
increasing the occupation ratio of (001).sub.KNN plane to 80% or
more.
Examples of Piezoelectric Thin Film Device Having Structure in FIG.
8
[0069] Examples of the piezoelectric thin film device having a
structure in FIG. 8 will be described below. FIG. 8 is a schematic
illustration showing a cross-sectional view of the piezoelectric
thin film device of Examples 5 to 8. As shown in FIG. 8, the
piezoelectric thin film device 80 of Examples 5 to 8 was fabricated
by sequentially forming, on an Si substrate 11 (having an SiO.sub.2
film 15 on its surface), a Ti adhesive layer 16, a Pt lower
electrode 12, an orientation control layer 17, a
(K.sub.0.5Na.sub.0.5)NbO.sub.3 piezoelectric thin film 13 and a Pt
upper electrode 14.
[0070] Next, the fabrication method of the piezoelectric thin film
device of Examples 5 to 8 will be described.
[0071] As the Si substrate 11, there was used the same conditions'
Si substrate with a thermal oxide layer 15 as was employed in
Examples 1 to 4. Firstly, on the Si substrate 11 was sequentially
formed the Ti adhesive layer 16 (thickness of 2 nm) and the Pt
lower electrode 12 (exclusively (111).sub.Pt oriented, thickness of
0.2 .mu.m) by RF magnetron sputtering. The formation conditions of
the Ti adhesive layer 16 and Pt lower electrode 12 were as follows:
substrate temperature of 400.degree. C.; discharge power of 200 W;
introduced gas of Ar; pressure of 2.5 Pa; and formation time of 2
min (for the Ti layer 16) and of 10 min (for the Pt electrode
12).
[0072] Subsequently, on the Pt lower electrode 12 was formed an
orientation control layer 17 (thickness of 200 to 300 nm) by RF
magnetron sputtering. As the orientation control layer 17, Examples
5 to 8 respectively used a 200-nm-thick LaNiO.sub.3 thin film,
300-nm-thick LaNiO.sub.3 thin film, 200-nm-thick NaNbO.sub.3 thin
film and 200-nm-thick (K.sub.0.2Na.sub.0.8)NbO.sub.3 thin film. The
condition of formation of the orientation control layer 17 was as
follows: sputtering target of sintered LaNiO.sub.3 (for Examples 5
and 6), sintered NaNbO.sub.3 (for Example 7) and sintered
(K.sub.0.2Na.sub.0.8)NbO.sub.3 (for Example 8); substrate
temperature of 600.degree. C.; discharge power of 100 W; introduced
gas of Ar; and pressure of 0.4 Pa.
[0073] Then, on the orientation control layer 17 was sequentially
formed a 3-.mu.m-thick (K.sub.0.5Na.sub.0.5)NbO.sub.3 thin film 13
and a 0.02-.mu.m-thick Pt upper electrode 14 by RF magnetron
sputtering. The formation conditions of the
(K.sub.0.5Na.sub.0.5)NbO.sub.3 thin film 13 and Pt upper electrode
14 were the same as those employed in Examples 1 to 4.
[0074] Similarly to Examples 1 to 4, the occupation ratio of
(001).sub.KNN plane of the KNN thin films 13 of Examples 5 to 8
were determined by the XRD 2.theta./.theta. measurement. Also,
their piezoelectric constants d.sub.31 were measured using the same
method as used in Examples 1 to 4.
[0075] FIG. 9 is a table showing, for Examples 5 to 8, type of an
orientation control layer, and occupation ratio of (001).sub.KNN
plane and piezoelectric properties of the KNN thin film. Further,
FIG. 10 shows a relationship between piezoelectric constant
d.sub.31 and applied electric field for piezoelectric thin film
devices of Examples 5 to 8.
[0076] As can be seen from Examples 5 to 8 in FIGS. 9 and 10, the
interposition of the orientation control layer 17 causes the
occupation ratio of (001).sub.KNN plane to be as strong as 93% or
more, and also further reduces the dependency of the piezoelectric
constant d.sub.31 on applied electric field[=|(d.sub.31 under 70
kV/cm)-(d.sub.31 under 7 kV/cm)|/|d.sub.31 under 70 kV/cm|] to as
low as 0.11 or less while still exhibiting a high d.sub.31 value
under 7 kV/cm.
[0077] Although the invention has been described with respect to
the specific embodiments for complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art which fairly fall within the
basic teaching herein set forth.
* * * * *